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Electronic Science

Digital Electronics 27. Digital System Design using PLDs

Module -27 Digital System Design

1. Introduction

2. Digital System Design

2.1 Standard (Fixed function) ICs based approach

2.2 Programmable ICs based approach

3. Comparison of Digital System Design approaches

4. Summary

Learning Outcome

After completing this module, you will be able to

1. Explain the importance of digital system design

2. Describe digital system design approaches

3. Understand the difference between fixed function ICs and Programmable logic devices

(PLDs)

4. Compare fixed function ICs and PLDs

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Electronic Science

Digital Electronics 27. Digital System Design using PLDs

1. Introduction

This module is dedicated to understand the implementation methods for digital system design. This

module is divided into four sections. In the introduction section, the importance of Digital System

Design will be discussed. In the second section, different desing approaches will be explored. There

are two main approaches: 1. Standard or fixed function ICs based approach and 2. Programmable

IC based approach. In the third section, different digital design approaches will be compared.

Whereas, last section will summarize the important take home points.

Digital systems have created a digital revolution everywhere. Digital convergence leads to

transformation and merging of various analog systems into digital world. The applications of digital

systems have entered in all areas of human activity right from use of computers, electronic gadgets,

house hold appliances, medical equipment to smart phones for internet access and e-purchases. One

of the important reasons for this growth of digital electronics could be the use of Integrated Circuits

(ICs) in all designs.

2. Digital System Design

Figure -1: Typical digital system

Digital design is the science of organizing information, defining operations, accepting inputs and

generating outputs using discrete quantities as shown in figure 1. The quality of products, processes

and services is increasingly dependent on digital system design. There are various techniques used

for designing combinational and sequential circuits. In addition, one can select technology options

i.e. TTL, CMOS, ECL etc. based on their relative advantages and disadvantages.

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Electronic Science

Digital Electronics 27. Digital System Design using PLDs

Traditionally, the digital system design methods were based on use of SSI and MSI circuits. These

traditional methods of design are now challenged by recent digital devices called Programmable

Logic Devices (PLDs), which are based on LSI circuits. The aim of this module is to provide an

overview of various design options that are available. The decision regarding the choice of design

technique would depend upon several parameters like power requirements, size, cost, design

complexity, frequency of operation etc.

Figure 1: Digital System Design Options

For designing the digital system for any application, the knowledge of various implementation

technologies is essential. Designer must decide whether to use dedicated (fixed) functionality digital

Digital System Design Option

Standard products

Application Specific ICs

Mask Programmable

Fixed function ICs

Field Programmable

Processor

Std. Cell

Gate Array

Full Custom

Microprocessor

PAL

ROM

Microcontroller

Digital Signal processor

4 XXX

74 XX

10 XXX

GAL

PLA

RAM

CPLD

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Digital Electronics 27. Digital System Design using PLDs

logic, or to use a software-programmed, processor-based system (designed based on a

microprocessor, microcontroller, or digital signal processor, DSP), or to use an application specific

ICs or hardware-configured programmable logic device (PLD), whether simple (SPLD), complex

(CPLD), or the field programmable gate array (FPGA). Memory used for the storage of data and

program code is integral to many digital circuits and systems as shown in figure 2.

2.1 Standard (Fixed function) ICs based approach:

In traditional design approach, a designer can select from the available digital ICs most

appropriate for the application, starting from a block diagram design concept and use available ICs.

The advantages of traditional approach (i.e. fixed-function ICs) are:

1. Shorter design cycle – Time require to design the system from concept to product is short.

2. Lower development cost – As the design is based on fixed function ICs, the development

cost is low.

3. Easier to test & troubleshoot – As the designer is involved in designing the complete

product, it is easy to test and troubleshoot.

Some of the disadvantages of traditional approach are

1. Larger space requirement – Fixed function ICs occupy finite space in the total board area.

2. Higher power requirements – As there are discrete components in the device every IC would

take finite power. The total required power is the sum of power consumption of each IC.

3. Hardwiring for interconnections - Modifications are difficult. The complete design is based

on discrete hardware. Their interconnections are hardwired on the board like PCB. On such

board modification are difficult.

4. Lesser reliability – Overall design involve many functional ICs along with separate

interconnection the reliability interconnection is low.

5. Lesser design flexibility – Due to complete hard-wiring in the design, there is less flexibility

for the design.

6. Difficult to Copyright – If one can carry out reverse engineering and trace the wiring

diagram along with every IC, functionally it is very difficult.

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Electronic Science

Digital Electronics 27. Digital System Design using PLDs

2.2 Programmable ICs based approach: There are two more approaches for designing the digital

systems: Use of application specific integrated circuits (ASICs) usually mask programmable and

use of programmable logic devices (PLDs) which are field programmable. All field programmable

devices are programmed in laboratory. However, mask programmable devices must be sent to a

manufacturer for implementation. The ASICs are developed to overcome the disadvantages of

design based on fixed function ICs. In ASICs, single IC is designed and manufactured to implement

logic functions. Many complex designs could be implemented easily using ASICs. The advantages

of ASICs are

1. Smaller space requirement (as integrated in single chip)- The programmable ICs normally

provide single chip solution hence the space requirement is small.

2. Lower power requirements – Single chip solution has lower power consumption. Therefore

overall power requirements are low.

3. Large volume production can bring the cost down. if this design is used in large volume

productive the cost of product can be brought down.

4. Easier to copyright – One can copyright the design easily as it is a single chip solution.

Major part of design is soft wired not hard wired.

5. Highest reliability – The interconnection density is very low, this design offer highest

reliability.

Some of the disadvantages of ASICs are:

1. Longer design and development cycle time – Usually ASICs are mask programmable hence

the design and development cycle is longer and takes more time.

2. Design and Development cost is high – As ASICs utilized specialized tools for the design

and development, the total cost is higher.

3. Special testing and verification tools are required – For all software based implementation

there is need to have special testing and verification tools for different ASICs.

In the third approach, programmable logic devices may be used to implement digital systems. The

PLDs have advantages of both the approaches as discussed above. Field programmable logic

requires both hardware and software. PLDs are basically user configurable and re-programmable

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Digital Electronics 27. Digital System Design using PLDs

logic devices. User can program or change the program as per the requirement of the application.

The main advantages are:

1. Lower design and implementation cost – As programming done in the lab.

2. Smaller space requirement (as integrated in single chip) – the single chip solution on

PLDs occupy smaller space on board.

3. Lower power requirements – Due to single chip solution the power requirement are low.

4. Higher reliability – As there are minimum interconnections on board, the reliability and

design is higher.

5. Easier to test – Usually simulation are carried out before programming it is easier to test.

6. Easier to copy right – Single chip solution can easily have copyright for the design.

7. More design flexibility – As PLDs reprogrammable and reconfigurable there is more

design flexibility.

There are certain disadvantages of PLDs:

• Inefficient use of logic devices and resources - If the design involves less product term

or sum terms then the AND & OR arrays are not utilize to their full capacity therefore,

PLDs do not use all logic devices efficiently.

• Larger number of logic gates for implementation – PLDs provide all possible product

terms using buffer/inverters at inputs hence there are large.

3. Comparison of Digital System Design approaches

Digital systems can be implemented by either fixed function ICs or programmable ICs. Digital

systems based on fixed function ICs make use of basic logic gates and others functions such as

adder, multiplexers/demultiplexer, encoders/decoders, counters, shift registers. These ICs are

designed by their manufacturer and produced in large quantities. On the other hand, programmable

ICs are designed and can be easily configured by the user. Comparison between fixed function ICs

and Programmable ICs is given in table 1.

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Digital Electronics 27. Digital System Design using PLDs

Table -1: Comparison of Digital System design approaches

Parameter Fixed function ICs Programmable ICs 1. Device Density Less More

2. Board Space Large Small

3. Power requirements Large Small

4. Development time Less More

5. Development cost Low High

6. Logic testing Simple Easy

7. Design complexity Less More

8. Design flexibility Less More

9. Design time Less Less

10. Design security Low High

11. Modification Difficult Easier

In order to compare digital system design approach, we will consider different parameters in first

column for comparison. The second column describes the characteristics and fixed function IC and

the third column describe the characteristics of programmable ICs design approach.

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Digital Electronics 27. Digital System Design using PLDs

4. Summary

Digital system design using standard or fixed parts are quick to realize and relatively easy to debug.

However, they are bulky and expensive when high volumes are required. The various functions are

available that allow all sorts of digital systems to be implemented with minimal overheads and

tools. Digital system design using ASICs or Custom ICs are most flexible option because, there are

very few constraints on how application specific logic is implemented.

……..